1. Field of the Invention
The present invention relates generally to digital modulator systems.
2. Description of the Related Art
Digital modulators find use in a variety of modern communication systems. Cable modems, for example, employ digital modulators to provide high-speed Internet service over bidirectional transmission systems (e.g., coaxial cables and optical fibers). Downstream data has typically been processed with quadrature amplitude modulation (QAM). Although quadrature phase-shift keying (QPSK) is a typical upstream modulation process, other processes (e.g., QAM) are often used to enhance the efficiency (bits/Hz) of the upstream data path. Cable modems have typically operated with an upstream data rate (e.g., 10 Mb/s) in an upstream communications band (e.g., 5-42 MHz) and with a higher downstream data rate (e.g., 36 Mb/s) in a different downstream communications band (e.g., 42-850 MHz).
Realizing digital modulators with extremely high carrier frequencies Fc is challenging because the Nyquist criteria requires that the system sampling rate Rs be at least twice the carrier frequency Fc. For a carrier frequency in the region of 500 MHz, a modem""s quadrature modulator must therefore operate at a system rate Rs of 1 GHz. Design of a 1 GHz modulator generally requires extensive pipelining of the modulator""s adders and multipliers with consequent increase of modulator costs.
The present invention is directed to digital modulator structures and methods that can operate at extended system rates Rs and thus realize high carrier frequencies Fc.
These goals are realized with modulator structures and methods that facilitate the use of M quadrature modulators which modulate, at a reduced modulation rate Rs/M, respective ones of M polyphase cosine elements and M polyphase sine elements with respective ones of interpolated I elements and interpolated Q elements to thereby form M polyphase modulated elements.
Modulator embodiments of the invention are thus significantly easier to realize for a high carrier frequencies Fc and their structure can be substantially simplified.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.